Fix for issue 664: https://github.com/docker/distribution/issues/664

Errors thrown by storage drivers don't have the name of the driver, causing user
confusion about whether the error is coming from Docker or from a storage driver.
This change adds the storage driver name to each error message.

This required changing ErrUnsupportedDriver to a type, leading to code changes
whenever ErrUnsupportedDriver is used.  The tests check whether the driver name
appears in the error message.

Signed-off-by: Amit Shukla <amit.shukla@docker.com>

docs/storage/blobserver.go

@@ -36,16 +36,15 @@ func (bs *blobServer) ServeBlob(ctx context.Context, w http.ResponseWriter, r *h
 
 	redirectURL, err := bs.driver.URLFor(ctx, path, map[string]interface{}{"method": r.Method})
 
-	switch err {
-	case nil:
+	if err == nil {
 		if bs.redirect {
 			// Redirect to storage URL.
 			http.Redirect(w, r, redirectURL, http.StatusTemporaryRedirect)
 			return err
 		}
+	}
 
-		fallthrough
-	case driver.ErrUnsupportedMethod:
+	if _, ok := err.(*driver.ErrUnsupportedMethod); ok {
 		// Fallback to serving the content directly.
 		br, err := newFileReader(ctx, bs.driver, path, desc.Size)
 		if err != nil {